Silicon-ceramic composite substrate

ABSTRACT

The invention relates to a silicon-ceramic composite substrate ( 01 ), which comprises a low-temperature ceramic ( 02 ) having at least one pre-structured ceramic layer and a silicon substrate ( 03 ). According to the invention, the low-temperature ceramic ( 02 ) forms a carrier layer and the silicon substrate surface has nanostructures ( 06 ), which are completely penetrated into the low-temperature ceramic ( 02 ), in a contact area ( 04 ) with the carrier layer.

The present invention concerns a composite substrate made from siliconand ceramic according to the preamble of claim 1.

The integration of semiconductor chips on ceramic carrier substrates,which can feature a wiring structure and passive electronic componentsand serves the purpose of an electrical connection of a complete system,is an established method. The bond between the completed semi-conductorchip and the ceramic carrier is usually realized by means of bondingmethods such as flip-chip bonds, different soldering techniques oradhesion methods with subsequent wire-bonding for the creation of theelectrical connection.

These connection techniques work however always with additionalauxiliary means such as metallization, soldering, or special adhesives,and usually require a high calibration and installation effort.

It is also known how to manufacture multi-layer circuits on the basis ofsintered ceramic carriers with low-temperature co-fired ceramics (LTCC).By these means conduction paths, resistors, coils or also fluid channelscan be produced. The elements can be applied by means of screen printingor photochemical processes. The green ceramic foils are individuallystructured, stacked thereafter, and laminated. Finally a definedsintering profile is executed with a peak temperature of about 850-900°C.

A more recent method is experimentally introduced in [C. Rusu et. al,LTCC interconnects in microsystems, Journal of Micromechanics andMicroengineering, 16 (2006), page 13-18].

It initially describes the possibility of connecting an LTCC ceramicwith silicon by means of anodic bonding The connection of a 2×2 cm²sized ceramic substrate with a silicon wafer by means of anodic bondingis introduced among other things. In this context a special lowtemperature ceramic (Low Temperature Cofired Ceramics—LTCC) is utilizedwhich permits, because of its consistency and an average to low thermalexpansion coefficient, the anodic bonding of the ceramic on silicon.Nonetheless the initially fired low-temperature ceramic must be preparedfor the bonding process through additional, elaborate process steps,such as grinding and polishing. Furthermore a device is required inaddition for the anodic bonding. Even small imperfections in the surfaceor deposited particles lead to gas enclosures that negatively affect thedurability of the connection.

A simple silicon-ceramic bond can be produced by means of laminating andsintering of the bond partners [M. Fischer et. al, Bonding of ceramicand silicon—new options and applications, Smart Systems Integration,2007] by initially laminating the green ceramic onto the nano-structuredsilicon surface. The sintering process follows subsequently. As a resultthe elaborate polishing of the ceramic is omitted.

It is the purpose of the present invention to provide a silicon-ceramiccomposite substrate of high mechanical strength and variablefunctionality that can be manufactured economically and free ofauxiliary materials and can be further processed with the known standardsemiconductor process technology.

The solution to this problem is provided according to the invention bymeans of a silicon ceramic composite substrate with the characteristicsaccording to patent claim 1 or 10.

The silicon-ceramic composite substrate according to the inventionencompasses a low-temperature ceramic with at least one pre-structuredceramic layer and a silicon substrate. The surface of the siliconsubstrate features nanostructures in a contact area with thelow-temperature ceramic that have completely penetrated into thelow-temperature ceramic. The low-temperature ceramic forms a carrierlayer for the silicon substrate which can be processed by means of knownsemi-conductor technologies.

Preferably the nanostructures are formed from so-called “black silicon”which features needle-like spikes.

It is however also possible, and part of the idea of the invention, toprovide in the contact area other nano-structures which can be producedfor example through polishing, etching or otherwise processing of thesilicon substrate surface.

The thermal expansion behavior of the low-temperature ceramic isadvantageously adapted to the expansion behavior of the siliconsubstrate, so that stress at the connection location is minimized.

For the manufacture of the silicon-ceramic composite substratesaccording to the invention green ceramic foils are initiallypre-structured in one or more layers, namely provided with conductionpaths, vias (plated through-holes), fluid channels, or also withresistors, capacitors and/or coils.

The pre-structuring is implemented with the aid of standard methods,such as for example punching, via-filling, screen printing, or lasertreatment. The ceramic foils are subsequently layered on top of eachother for the manufacture of a desired functionality or for themanufacture of a carrier layer.

A nano-structure is applied onto the silicon substrate. Thereafter thesilicon substrate is laminated on the carrier layer subject to theeffect of temperature and pressure and the composite so created issintered.

Since the utilized bonding mechanism distinguishes itself from anodicbonding, one can advantageously forgo the sodium-containing glasses thatare required for the anodic bonding of the low-temperature ceramics andgenerate a semiconductor process-compatible composite substrate thatforms the basis for different MEMS (Micro-Electro-Mechanical System)applications.

The advantages of the invention are particularly to be found in the factthat a composite substrate is presented that distinguishes itselfthrough high multi-functionality and high mechanical strength. Inaddition different-type functionalities can be integrated into thecarrier layer by means of the modified LTCC technology.

It has proven to be particularly advantageous that one can, to beginwith, forgo the entire operation of the sintering of the LTCC ceramicsince this occurs integrally with the manufacture of the silicon ceramiccomposite substrate according to the invention. In addition it ispossible, due to the elimination of the otherwise initially requiredsintering process of the LTCC, to achieve a larger plurality of LTCCstructures that can be manufactured since these can be assembled aslayers of green ceramic foils based on demand.

Additional preferred embodiments of the invention are provided in thesub claims.

Additional details and advantages of the invention can be discerned fromthe description part that follows in which the invention is furtherexplained in reference to the enclosed drawings.

The drawings show:

FIG. 1-a schematic representation of the process steps for themanufacture of a silicon ceramic composite substrate according to theinvention.

FIG. 2-a schematic representation of the manufacture of an electricalcontact in a silicon ceramic composite substrate.

FIG. 3-a schematic representation regarding the separation of siliconchips on a silicon ceramic composite substrate.

FIG. 1 presents a schematic representation of the process steps for themanufacture of a silicon ceramic composite substrate according to theinvention.

Onto a green low-temperature ceramic 02, which consists of one orseveral ceramic layers and can already feature a wiring structure withconduction paths, plated through-hole (vias), resistors, coils,capacitors and fluidic channels (not represented), a silicon substrate03, which features in a contact area 04 a nanostructure 06, is laminatedsubject to the effect of pressure and temperature without the additionof auxiliary means. The silicon substrate 03 is preferably a siliconwafer that is completely or partially nano-structured.

The nano-structure 06 of the surface of the silicon substrate 03 canthereby be realized for example by means of a self-maskingplasma-etching process, whereby the geometric dimensions of theneedle-like nano-structures 06 produced thereby are preferably adaptedto the powder morphology of the green low-temperature ceramic 02 (forexample the kernel size of the solids of the raw ceramic). A preferableneedle structure thereby features spacings of the needles that are inthe range of the kernel size of the solids of the raw ceramic.

The lamination is implemented in a press (indicated by the arrows 07),for example at temperatures between 80° C. and 120° C. in a time span of1 to 30 minutes.

Subsequently the firmly bonded and/or positively locking silicon ceramiccomposite substrate 01 is manufactured preferably in a pressuresintering process (indicated by the arrow 08) at maximal temperatures ofup to 950.

The needle-like nano-structures 06 penetrate during lamination 07completely into the low-temperature ceramic 02 and effect thereby atight bond that for example permits fluid channels that are present inthe low-temperature ceramic to be guided along the silicon. This can beutilized advantageously for the cooling of the structure that is createdlater in the silicon. The cooling channels that are generated therebyare particularly effective in regard to the silicon because of thesurface area that is enlarged due to the nano-structure 06.

FIG. 2 presents a schematic representation of a preferred embodiment ofthe invention. If prior to the lamination a metallization 09 is appliedto the needles of the nano-structure 06, an electrical bond can beestablished during the sintering between the conduction paths that arepresent on the low-temperature ceramic 02 (not presented) or metallicvias 11 and the metallization 09 that is present on the siliconsubstrate 03.

The silicon ceramic composite substrate 01 that is thereby created,which distinguishes itself through its very high strength, preferablyfeatures the outer contour of a standard wafer (for example 4″) and istherefore compatible with all installations and devices for subsequentsemi-conductor processing (lithography, thin-film techniques, plasmastructuring methods etc.).

If the ceramic-silicon composite substrate 01 according to the inventionis manufactured in the described fashion and further processed, thesilicon substrate 03 itself does not have to feature high mechanicalstrength since the low temperature ceramic 02 takes on the carrierfunction during subsequent technological follow-on steps. This meansthat the silicon substrate 03 has to be implemented only so thick as toassure the electronic functions, which again leads to significantmaterial savings. Furthermore the process time for example for etchingcan be significantly reduced if the silicon layer features only aminimum thickness.

An advantageous thickness of the silicon substrate 03 is approximatelyin the range of 50 to 100 micrometers.

Similarly to the case of SOI (Silicon On Insulator) technology siliconis applied only where it is functionally required.

In FIG. 3 is presented that functional areas, which are defined duringfurther processing, can be separated on the silicon ceramic compositesubstrate 01, such as in the case here chips 12 in the silicon plane.This can be accomplished by means of standard silicon etching processes(for example reactive ion etching—DRIE), whereby the surface of thelow-temperature ceramic 02 functions as a natural etching stop.

The standard technologies for preparation and processing of thelow-temperature ceramic permit an economical integration of electrical,fluidic, and optical interfaces to the periphery.

For the manufacture of the silicon ceramic composite substratesaccording to the invention no additional devices, such as chip bondersor bond installations for anodic bonding, are required. One canmanufacture with the installations that are present anyhow for thestandard processes of the semiconductor and LTCC technology.

LIST OF REFERENCE SYMBOLS

-   01 Silicon-Ceramic Composite Substrate-   02 Low-temperature ceramic-   03 Silicon substrate-   04 Contact area-   05 --   06 Nano-structure-   07 Laminate-   08 Sintering-   09 Metallization-   10 --   11 Via-   12 Chip

1. Silicon ceramic composite substrate (01) encompassing alow-temperature ceramic (02) with at least one pre-structured ceramiclayer and one silicon substrate (03), characterized in that thelow-temperature ceramic (02) forms a carrier layer and the surface ofthe silicon substrate (03) features nano-structures in a contact area(04) to this carrier layer which have penetrated completely into thelow-temperature ceramic (02).
 2. Silicon ceramic composite substrate(01) according to claim 1, characterized in that the geometricdimensions of the nano-structure (06) are adapted to the powdermorphology of the low-temperature ceramic (02).
 3. Silicon ceramiccomposite substrate (01) according to claim 1 or 2, characterized inthat the nano-structures (06) are needle-shaped and the spacing betweenthe needles corresponds to the kernel size of the powder-shapedcomponents of the low-temperature ceramic (02).
 4. Silicon ceramiccomposite substrate (01) according to any of the claims 1 to 3,characterized in that the low-temperature ceramic (02) features vias(11), conduction paths and/or fluid channels.
 5. Silicon ceramiccomposite substrate (01) according to any of the claims 1 to 4,characterized in that the surfaces of the nano-structures (06) featurecomplete or partial metallization (09).
 6. Silicon ceramic compositesubstrate (01) according to claim 5, characterized in that an electricconnection between the metallization (09) of the silicon substrate (03)and the conduction paths and/or vias (11) exists.
 7. Silicon ceramiccomposite substrate (01) according to any of the claims 1 to 6,characterized in that it features the shape of a standard wafer and isfurther processable by means of semi-conductor technologies.
 8. Siliconceramic composite substrate (01) according to any of the claims 1 to 7,characterized in that the silicon substrate (03) is implemented only sothick as to make the electronic functions that are to be realized on itmanufacturable.
 9. Silicon ceramic composite substrate (01) according toany of the claims 1 to 8, characterized in that the silicon substrate(03) features a thickness of 50 to 100 micrometers.
 10. Thesilicon-ceramic composite substrate (01) encompassing a low-temperatureceramic (02) with at least one pre-structured ceramic layer and asilicon substrate (03), obtained through a process with the followingsteps: Structuring of a green ceramic foil; Stacking of one or severallayers of the green ceramic foil up to a carrier layer; Manufacture of anano-structure on the surface of a silicon substrate; Laminating of thesilicon substrate with the nano-structured surface on the carrier layersubject to the application of pressure and temperature and therebycomplete penetration of the nano-structure into the carrier layer. 11.Silicon ceramic composite substrate (01) according to claim 10,characterized in that it is subsequently subjected to a pressuresintering process at maximal 950° C.
 12. Silicon ceramic compositesubstrate (01) according to claim 10 or 11, characterized in that duringthe structuring of the ceramic foil conduction paths, vias and/or fluidchannels are produced in or on the same.
 13. Silicon ceramic compositesubstrate (01) according to claims 10 to 12, characterized in thatduring the structuring of the ceramic foil resistors, capacitors, and/orcoils are produced in or on the same.